The present invention relates, in an aspect of the invention, to a method for diagnosing of the functionality of at least one NOx-adsorbing catalyzer (for example of the LNA type) during operation onboard a vehicle, which catalyzer is arranged in a vehicle comprising a combustion engine that, during operation, emits exhaust gases to an exhaust emission control system comprising said catalyzer.
The present invention also relates, in an aspect of the invention, to a engine-driven vehicle in which the above mentioned method is utilized and that, in addition, comprises a combustion engine that, during operation, emits exhaust gases to an exhaust emission control system comprising at least one NOx-adsorbing catalyzer, an injection device for injecting a reductant into the exhaust emission control system upstream of said catalyzer and a control unit for controlling at least the injection device.
The present invention also relates, in an aspect of the invention, to a computer program product containing a computer program intended for carrying out such a method using a computer.
Legislation concerning diesel engines has been made more stringent and will be made even more stringent in the future, in particular with regard to the emission of nitric oxide pollutants and particulates.
The quantity of nitric oxides that is created during the combustion of fuel in the cylinder of an engine is dependent upon the temperature at which the combustion takes place. Higher temperatures result in a greater proportion of the nitrogen in the air being converted to nitric oxides (NOx). The catalyzers that are used on diesel engines and other engines that operate with an excess of air are mostly oxidizing. As the exhaust gases contain oxygen, it is difficult to reduce the nitric oxides in a selective way. In addition to nitric oxides, unwanted emissions are also produced during the combustion process, including carbon monoxide (CO), hydrocarbon (HC) and particulates, primarily in the form of soot (C).
A known method for reducing the quantity of nitric oxides that is based on exhaust emission control is an LNA NOx trap (Lean NOx Adsorber). An LNA can also be called an LNT (Lean NOx Trap) or NAC (NOx Adsorber Catalyst). The technology is based on NO first being oxidized to NO2 in an oxidizing catalyzer, after which the NO2 is adsorbed in the trap in the form of nitrates. The adsorption of NO2 takes place when the engine is working with an excess of oxygen. The regeneration of the NOx trap takes place intermittently at predetermined intervals by letting the engine operate with a deficiency of oxygen, that is with an extra addition of hydrocarbon (reductant) and/or reduced airflow, which destabilizes the nitrates and reduces the nitrogen dioxide NO2 trapped in the NOx trap to nitrogen and water H2O. See for example U.S. Pat. No. 5,473,887 or U.S. Pat. No. 6,718,757. Both the adsorption and the regeneration require the temperature in the NOx trap to be sufficiently high (in excess of 2000 C for adsorption and approximately 3000 C for regeneration). With low loads on the engine (for example, driving in towns or an unloaded goods vehicle), the exhaust gas temperature will not be sufficient to keep the NOx trap at the required temperature. One way of forcing the temperature to suitable levels is to inject hydrocarbon (and/or CO or H2) into the exhaust gases that are then subjected to catalytic combustion in the NOx trap so that the correct temperature is attained.
The addition of a reductant can be carried out by an extra injection (post-injection) at the open exhaust gas valve in the engine or via an injector arranged on the exhaust pipe.
If, for some reason, the LNA has an unforeseen reduced NOx adsorption functionality, then an increased quantity of NOx will be released out into the atmosphere. For example, dependent upon the sulfur content (S) of the fuel, the NOx-reducing catalyzer will be sulfur poisoned sooner or later. Sulfur poisoning means that the sulfur forms sulfates that reduce the ability of the NOx-reducing catalyzer to adsorb nitrates. It is not possible to remove the sulfates by means of the abovementioned NOx regeneration process. In order to clean adsorbed sulfur from the NOx-reducing catalyzer, a so-called deSOx (sulfur regeneration) is performed, which involves a reduction of lambda and an increase in the temperature in the LNA to 600-7000 C. The carrying out of a deSOx is a known technique (see for example EP 1500800). A deSOx is not carried out as frequently as a NOx regeneration, but, on account of the high temperature, the procedure leads to a significant increase in the consumption of reductant, which means that, in those cases where the vehicle's fuel is used as the reductant, there is a significant increase in the fuel consumption.
In order to ensure that the functionality is maintained and hence that the requirements laid down in legislation are fulfilled, diagnosing can be carried out-of parts or the whole of the exhaust emission control system onboard and during operation of the vehicle.
EP 1174601 shows an example of a method for diagnosing of an exhaust emission control system based on measurements of temperature during operation of the vehicle. A predetermined quantity of hydrocarbon HC is injected periodically. The exotherm is measured by a temperature sensor, that is a recording of the “light-off” temperature is carried out, and on the basis of the measured temperature values, it is determined whether the exhaust emission control system has reduced functionality or not.
U.S. Pat. No. 6,519,930 shows an arrangement for NOx adsorption monitoring, testing and determination of the functionality of a NOx-reducing catalyzer by measuring the temperature before and after a predetermined regeneration of the NOx-reducing catalyzer. The difference between the measured temperature values provides an indication of the performance of the NOx-reducing catalyzer.
In a NOx trap, in addition to NOx, sulfates can also be adsorbed (see above) in various quantities depending upon the sulfur content in the fuel. A disadvantage of the known technology is that there is no satisfactory diagnosis procedure for this with associated program of remedial measures. Another disadvantage of the known technology with diagnosis procedures based on the measurement of temperatures is that it takes time for the heat from the regeneration to spread through the material and to reach the temperature sensor. The measurement of the temperature thus takes place with a certain calculated delay, which results in a major uncertainty for a system based on the measurement of temperatures. A more direct way of determining the exotherm is to measure the drop in pressure across the catalyzer. Measurement of the drop in pressure is known through, for example, U.S. Pat. No. 3,766,536.
U.S. Pat. No. 3,766,536 shows a catalyzer (not of LNA type) that can have NOx-reducing functionality. The drop in pressure across the catalyzer is measured continually during the operation of the vehicle by pressure sensors. When the measured drop in pressure is less than a certain predetermined value, this is an indication that the functionality of the catalyzer is reduced to some extent. This does not include any further monitoring and remedial measures concerning the functionality of the catalyzer. The device according to U.S. Pat. No. 3,766,536 can only provide an indication that the functionality of the catalyzer is reduced.
There are also solutions with a so-called NOx sensor, that is where the NOx content is measured directly after the LNA. A NOx sensor is, however, relatively expensive and relatively likely to go wrong.
It is desirable to carry out diagnosing of the functionality of at least one NOx-adsorbing catalyzer onboard and during operation of the vehicle and, in the event of finding a reduction in the performance of the catalyzer, to take measures by means of predetermined steps to try to improve the performance of the catalyzer automatically onboard and during operation of the vehicle with the aim of further minimizing unwanted overall exhaust gas emissions. In exhaust emission control systems with bypass pipes and bypass valves, it is also desirable to carry out diagnosing of the bypass valve. It is also desirable in exhaust emission control systems with clean-up catalyzers to carry out diagnosing of the clean-up catalyzer.
The method according to an aspect of the invention comprises a method carried out during operation onboard a vehicle for diagnosing of at least a part of an exhaust emission control system arranged in a vehicle with a combustion engine that emits exhaust gases during operation to said exhaust emission control system. The exhaust emission control system comprises at least one NOχ-reducing catalyzer that can be regenerated and, during at least a predetermined period of time, a predetermined quantity of reductant is added to the exhaust emission control system upstream of at least the NOx-reducing catalyzer for regeneration of the NOx-reducing catalyzer. A drop in pressure across the NOx-reducing catalyzer is measured. The method is characterized by the following steps:
a first measurement and recording is carried out of a first measured value for the drop in pressure at a point in time immediately before the reductant is added to the exhaust emission control system;
a second measurement and recording is carried out of a second measured value for the drop in pressure at a point in time after the completion of the regeneration;
a comparison is carried out between said first and second values, after which a deSOx is carried out if the increase in the drop in pressure between the first and second measured values is less than a first predetermined value.
An advantage that is obtained with the method according to an aspect of the invention is that a deSOx is only carried out when it is actually necessary. In this way, the consumption of reductant is minimized. In an embodiment with an exhaust emission control system with double NOx-reducing catalyzers arranged in parallel, the consumption of reductant can be reduced still further.
An aspect of the invention also comprises a device in the form of a engine-driven vehicle with an exhaust emission control system where there is diagnosing of the NOx-reducing catalyzer according to the present invention. The same advantages are obtained with the device according to an aspect of the invention as with the method according to an aspect of the invention.
In an alternative embodiment of the method and the device according to an aspect of the invention, a bypass pipe is arranged to take the exhaust gases past said NOx-reducing catalyzer when required and a three-way valve is arranged to direct all or parts of the exhaust gases via said NOx-reducing catalyzer or via said bypass pipe. A clean-up catalyzer is arranged downstream of the NOχ-reducing catalyzer and the bypass pipe. According to an aspect of the invention, in addition to the abovementioned method steps, a control unit is arranged also to carry out the following steps:
the three-way valve is moved to a predetermined position where a partial flow of the exhaust gases is directed in through the NOx-reducing catalyzer and the remaining flow of the exhaust gases is taken past the NOx-reducing catalyzer through the bypass pipe;
a greater quantity of reductant is injected in comparison with what is required for a satisfactory NOx regeneration;
a measurement and recording is carried out of an increase in temperature in the clean-up catalyzer; a comparison is carried out between said measured increase in temperature and a stored reference value for the minimum acceptable increase in temperature in the clean-up catalyzer, with a fault code being generated if the measured increase in temperature is less than said reference value for the minimum increase in temperature.
The advantage of this embodiment is that, in exhaust emission control systems with clean-up catalyzer, diagnosing of the clean-up catalyzer can be carried out using the same injection devices for reductant that are used for regeneration of the NOx-reducing catalyzer.
In yet another alternative embodiment of the method and device according to an aspect of the invention, an additional NOx-reducing catalyzer comprised in the exhaust emission control system is arranged on the bypass pipe. The control unit is here arranged to use each said NOx-reducing catalyzer, with associated part of the exhaust pipe, alternately as a bypass pipe for diagnosing of the clean-up catalyzer. Thus, the system does not need to use the same exhaust pipe with associated NOx-reducing catalyzer each time as a bypass for diagnosing of the clean-up catalyzer.
In another alternative embodiment of an aspect of the invention, the control unit is arranged to record at least a second increase in the drop in pressure based on the difference between a third measured value and said first measured value for the drop in pressure across the NOχ-reducing catalyzer, with a fault code being generated only if said second increase in the drop in pressure is also less than said first predetermined value. By carrying out at least two test cycles, a better statistical basis is obtained for the diagnosis.
In yet another alternative embodiment of an aspect of the invention, the degree of aging of the NOx-reducing catalyzer is recorded and compared with stored values for the corresponding minimum quantity of reductant that is required to obtain a satisfactory NOx regeneration for the NOx-reducing catalyzer. The control unit is here arranged to select the quantity of injected reductant according to said stored value corresponding to the aging of the NOx-reducing catalyzer. This provides the advantage of additional savings in the consumption of reductant.
In an alternative embodiment of the method and the device according to an aspect of the invention, this comprises a combustion engine that, during operation, emits exhaust gases to an exhaust emission control system, in which the exhaust emission control system comprises at least one NOx-reducing catalyzer that can be regenerated, a bypass pipe arranged to take, when required, the exhaust gases past said NOx-reducing catalyzer and a three-way valve arranged to direct all or parts of the exhaust gases via said NOx-reducing catalyzer or via said bypass pipe, injection devices arranged upstream of the NOx-reducing catalyzer and downstream of the three-way valve for injecting a reductant into the exhaust emission control system, a pressure sensor for measuring the drop in pressure across the NOx-reducing catalyzer, a control unit for recording the signal from the pressure sensor and for control of at least the injection device. An aspect of the invention is characterized in that the control unit is arranged to: move the three-way valve to an expected predetermined position according to control signals for controlling the three-way valve; record a measured value for the drop in pressure at a point in time during the time that the reductant is being added to the exhaust emission control system; compare said measured value for the drop in pressure and a stored value that corresponds to a drop in pressure with a correctly functioning three-way valve; and is characterized in that the control unit is arranged to generate a fault code for the three-way valve if the difference between said measured value and the stored value is greater than a first predetermined value.
The pressure sensor can thus also be used for diagnosing of a three-way valve in an exhaust emission control system with a bypass pipe or other parallel pipes. By using the same components for several diagnosis purposes, the diagnosis system can be made more efficient for a complete exhaust emission control system. In an alternative embodiment, one and the same regeneration is used for both diagnosing of the three-way valve and of the NOx-reducing catalyzer.
In an alternative embodiment of the method and the device according to an aspect of the invention, diagnosing according to an aspect of the invention (see above) of the three-way valve and the clean-up catalyzer is combined in one and the same exhaust emission control system. An advantage of this is that parts of the diagnosing of the three-way valve can be carried out at the same time as the diagnosing of the clean-up catalyzer. In an alternative embodiment comprising said diagnosing of the three-way valve and of the clean-up catalyzer, an additional NOx-reducing catalyzer comprised in the exhaust emission control system is arranged on the bypass pipe in the same way as described above. In this way, the control unit can also here be arranged to use each said NOx-reducing catalyzer, with associated part of the exhaust pipe, alternately as a bypass pipe for diagnosing of the clean-up catalyzer or of the three-way valve.
In yet another alternative embodiment of the method and device according to an aspect of the invention, the diagnosing according to an aspect of the invention (see above) of the three-way valve, the clean-up catalyzer and the NOx-reducing catalyzer are combined in one and the same exhaust emission control system. In this way, a relatively complete diagnosis system is obtained for all the components comprised in the exhaust emission control system and for the most important components. An embodiment of an aspect of the invention includes both double NOx-reducing catalyzers arranged in parallel (as described above) and all the diagnosing according to an aspect of the invention, that is diagnosing of the three-way valve, the clean-up catalyzer and the NOx-reducing catalyzer. In this embodiment, the greatest possible benefits are obtained as a result of coordination of the components that are required for the respective type of diagnosis. In addition, in this embodiment, the greatest possible effect is obtained with the smallest number of components with which all the abovementioned diagnosing can be carried out in an exhaust emission control system.
According to an embodiment of an aspect of the invention, the diagnosing of the three-way valve can be carried out after the combustion engine has been switched off for a predetermined number of hours (cold start). This is in order to obtain steady state conditions for carrying out the diagnosis with more reliable diagnosis results.